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As a source of heat load on cryogenic sections, the electron cloud is currently a major limitation to the intensity of some modern particle accelerators such as the LHC and its high luminosity upgrade at CERN. During LHC operation, conditioning of the copper beam pipe surface occurs, leading to a decrease of the cloud intensity. To understand the role of the different chemical surface components of air exposed copper in the electron conditioning process, air exposed copper samples as well as specific model surfaces produced in the laboratory, namely sputter-cleaned copper and carbon-free cuprous oxide (Cu 2 O), were conditioned by low energy electron irradiation. Conditioning of air exposed copper results in a decrease of the maximum secondary electron yield (SEY) below 1.1. Surface cleaning by electron stimulated desorption and carbon graphitization without increase of the carbon surface concentration are observed by x-ray photoelectron spectroscopy. After conditioning, the maximum SEY of both sputter-cleaned copper and Cu 2 O remains higher than 1.1. No significant surface modification is observed by x-ray photoelectron spectroscopy during irradiation for these two surfaces. These results prove that neither an increase of the amount of surface carbon nor oxide modification is responsible for the SEY reduction observed during electron irradiation of air exposed copper. They confirm that graphitic carbon is required to decrease the maximum SEY of copper below 1.1.

The surface resistance of state-of-the-art REBa2Cu3O7−x coated conductors has been measured at 8 GHz versus temperature and magnetic field. We show that the surface resistance of REBa2Cu3O7−x strongly depends on the microstructure of the material. We have compared our results to those determined by the rigid fluxon model. The model gives a very good qualitative description of our data, opening the door to unravel the effect of material microstructure and vortex interactions on the surface resistance of high temperature superconductors. Moreover, it provides a powerful tool to design the best coated conductor architecture that minimizes the in-field surface resistance. We have found that the surface resistance of REBa2Cu3O7−x at 50 K and up to 9 T is lower than that of copper. This fact poses coated conductors as strong candidate to substitute copper as a beamscreen coating in CERN’s future circular collider. To this end we have also analyzed the secondary electron yield (SEY) of REBa2Cu3O7−x and found a compatible coating made of sputtered Ti and amorphous carbon that decreases the SEY close to unity, a mandatory requirement for the beamscreen chamber of a circular collider in order to prevent the electron-cloud phenomenon.

In LHC the electron cloud induced by photoelectrons, gas ionization and secondary electrons emitted from the beam pipe walls could be a limitation of the performance. The electron cloud induce heat load on the cryogenic system, cause pressure rise, emittance growth and beam instabilities, which in the end will limit the beam"s lifetime. Beam-induced multipacting, which can arise through oscillatory motion of photoelectrons and low-energy secondary electrons bouncing back and forth between opposite walls of the vacuum chamber during successive passage of proton bunches, represent therefore a potential problem for the machine.The secondary electron yield (SEY) is one of the key parameters for the electron cloud build up and multipacting phenomenon. An electron cloud occurs if the metal surface secondary electron yield is high enough for electron multiplication. This parameter has been extensively studied on room temperature samples but uncertainties remain for samples at cryogenic temperature. Indeed, at low surface temperature SEY is strongly dependent on the nature of the physisorbed gases and on the surface coverage.In this work the secondary electron yield (SEY) at cryogenic temperatures has been measured and the results are presented. Of particular interest is the variation of the SEY with the gas coverage as most gases especially CO, CO 2 , and CH 4 in case of LHC condense on the cryogenic parts of the machine. In addition to these gases measurements have been performed with N 2, because of its importance as calibration gas for most of the pressure gauges and pumps. Also measurements with Kr have been done, to compare the results with existing works. In order to acquire a better understanding about the behaviour of SEY as a function of different gas coverage, measurements of work function have been made. The measurements of the SEY at cryogenic surfaces require a UHV system with a sample holder which can be cooled down to cryogenic temperatures, a source of primary electrons and detection of sample current and secondary electron current. In addition a gas injection system is necessary to produce variable coverage with different gases. In the present system the sample (Cu, Al, electro-polished Cu, Nb and C) was cooled down to 4.7K, irradiated with an electron gun and currents were measured on the sample and on a collector electrode. Work function was measured with a Kelvin Probe. KURZFASSUNG